The present invention relates to semiconductor processing in general and in particular to a system and process for reducing side lobes in resist coatings patterned with attenuated phase shift masks.
In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these high densities there have been, and continue to be, efforts toward scaling down device dimensions on semiconductor wafers. In order to accomplish higher device density, smaller and smaller features sizes are required. These may include the width and spacing of interconnecting lines, spacing and diameter of contact holes, and surface geometry of comers and edges of various features.
High resolution lithographic processes are used to achieve small features with close spacing between adjacent features. In general, lithography refers to processes for pattern transfer between various media. Lithography is a technique used for integrated circuit fabrication in which a silicon slice, the wafer, is coated uniformly with a radiation-sensitive film, the resist. The film is selectively exposed with radiation (such as optical light, x-rays, or an electron beam) through an intervening master template, the mask, forming a particular pattern. Areas of the coating exposed through the mask become either more or less soluble (depending on the type of coating) in a particular solvent developer. The more soluble areas are removed in a developing process leaving the less soluble areas in the form of a patterned coating.
When the mask pattern is sufficiently fine, diffraction of radiation at the edges of the pattern affects the lithographic process. As illustrated in FIG. 7 the intensity profile 700 of radiation passing through a narrow slit in an ordinary mask has a Gaussian distribution when the radiation reaches the substrate surface. As a result, edges of features are tapering rather than sharp and the resist image differs from that of the mask.
Attenuated phase shift masks (PSMs) are designed to compensate for diffraction and generate patterned resists with sharp edged features. Whereas the opaque regions of an ordinary mask are wholly opaque, the opaque regions of a PSM are partially transparent. The radiation that passes through the partially opaque regions, typically about 4 to about 15 percent of the incident radiation, is shifted in phase so that it is 180 degrees out of phase with the radiation passing through the transparent regions. The phase shifted radiation destructively interferes with diffracted radiation around the edges of transparent regions, giving the intensity profile 710 shown at the bottom of FIG. 7. Intensity profile 710 goes to zero near feature edges. However, the radiation intensity rises again away from the feature edges. As illustrated in FIG. 8, the intensity profile produced by a PSM mask 810 results in lobes near the edges of features and dimpling of the patterned resist 820 away from the feature edges. Side lobes and dimpling can lead to defects in the finished product. There is an unsatisfied need for a process of removing side lobes and dimpling from patterned resist coatings.
In invention provides a system for reducing or eliminating side lobes in patterned resist coatings. The system heats the resist briefly to induce the resist to flow. The system allows the resist to flow long enough for the side lobes to level, but not so long as to corrupt the resist pattern. The original resist pattern may be biased to allow for some flow during the side lobe reduction process. The invention is useful in eliminating side lobes that typically result when an attenuated phase shift mask is used to form a patterned resist coating with fine, sharp-edged features.
In one aspect, the invention provides a system for reducing side lobes in a patterned resist coating including a heating system that heats the resist in a controlled manner causing the resist to flow sufficiently to significantly reduce side lobes while substantially preserving the resist pattern.
In another aspect, the invention provides a system for reducing side lobes in a patterned resist coating on a substrate, including means for heating the resist in a controlled manner to cause the resist to flow sufficiently to substantially reduce side lobes while substantially preserving the resist pattern.
In another aspect, the invention provides a method of reducing side lobes in a patterned resist coating, including the step of applying sufficient heat to the resist to cause the resist to flow and thereby substantially reduce side lobes in the resist coating.
In a further aspect, the invention provides a method of preparing a patterned resist coating including the steps of forming a resist coating on a substrate, forming a first pattern in the resist using an attenuated phase shift mask, wherein the first pattern has side lobes, and applying heat to the resist and thereby causing it to flow until the side lobes are reduced.
The invention extends to features hereinafter fully described and features particularly pointed out in the claims. The following detailed description and the annexed drawings set forth in detail certain illustrative examples of the invention. These examples are indicative of but a few of the various ways in which the principles of the invention may be employed. Other ways in which the principles of the invention may be employed and other objects, advantages and novel features of the invention will be apparent from the detailed description of the invention when consider in conjunction with the drawings.